In cutting work of a pin journal that is an eccentric part of a crankshaft, so-called lathe-turning work in which a pin journal of the crankshaft is rotated with its axis and a main shaft center of a working device being matched and lathe-cutting is performed with a cutting tool, and milling work by means of a crankshaft miller are conventionally adopted generally. As a lathe-cutting machine of the former, which performs lathe-cutting, for example, a lathe-cutting machine for a crankshaft disclosed in Japanese Patent No. 2589971 is known, and this will be explained with reference to FIG. 9. FIG. 9 is a view in which an installment state of the crankshaft is added to an explanatory view of a chuck device of the crankshaft lathe-cutting machine described in the same Patent.
At both end portions of the above-described lathe-cutting machine, it has a chuck 142 which supports a crankshaft 100, rotationally drives a pin journal 102, and performs phase determination of rotational angle of the pin journal 102. A main operation of the chuck will be explained. Main journal parts at both ends of the crankshaft 100 are supported by chuck centers 175 and ternary claws 159, which are provided at the left and the right chucks 142. A shaft center A of the pin journal 102 to be worked and a center B of a main shaft 103 of the lathe-cutting machine are matched, and they are rotated around the axis of the pin journal 102 by a rotational drive device (not shown) at the rear of the main shaft 103. Lathe-cutting of the pin journal 102 is performed by means of a tool (not shown) provided at a tool stand (not shown) movable in an axial direction and a shaft diameter direction of the pin journal 102.
The crankshaft 100 shown in the drawing is an example for a four-cylinder engine. In this case, after a pin journal 102a is worked, setup change (so-called pin journal rotational angle phase indexing) to match a shaft center D of a pin journal 102b to the center B of the main shaft 103 is necessary to work the pin journal 102b different in phase by 180 degrees from the pin journal 102a. The pin journal rotational angle phase indexing is performed by releasing or fastening a coupling 144 by an indexing cylinder 147 with the crankshaft 100 being chucked by the ternary claw 159. The crankshaft 100 is rotated 180 degrees around a shaft center C of the crank shaft 100 with the chuck 142 by the rotational drive device (not shown) at the rear of a chuck drive shaft 105 via the chuck drive shaft 105 and a double link joint 141, and the rotational angle phase is indexed, whereby the pin journal rotational angle phase indexing is carried out.
Next, in the crankshaft working, it frequently happens that crankshafts differing only by a half stroke are fed to the same working machine at random, and therefore it is demanded to perform setup change by a half stroke quickly. A half stroke is the distance between the shaft center A of the pin journal 102 and the shaft center C of the crankshaft 100. The half stroke setup change is constituted to be performed by moving a slide 119, which is fixed to the chuck 142 by a predetermined half stroke by a hydraulic cylinder (not shown) with respect to the main shaft 103 on a plane including the shaft center A of the pin journal 102 and the shaft center C of the crankshaft 100.
However, by this half stroke conversion, the chuck 142 and the crankshaft 100 are away from and close to the center B of the main shaft 103, and unbalance amount of rotation around the center B of the main shaft 103 is increased and decreased, and is not fixed. To eliminate the unbalance amount, a balance weight 131 is provided in this machine, and this is structured to be interlocked with a moving amount corresponding to the half stroke conversion of the above-described slide 119 by the drive device (not shown) and moved in the opposite direction to the movement of the above-described slide 119.
As an example of a prior art of milling work by a crankshaft miller of the latter, there are many examples described in Japanese Patent Laid-open No. 8-25103, Japanese Patent Laid-open No. 11-90717 and the like. Each of them is a method of working an eccentric part (for example, a pin journal) of a crankshaft into a predetermined shape by milling work in which a cutting edge of a rotated milling cutter is moved in a predetermined locus within a plane perpendicular to an axis of an eccentric part of a crankshaft being a part to be worked
However, the prior art of the lathe-cutting machine for the crankshaft described in Japanese Patent No. 2589971 has the following problems.
(1) Since the crankshaft 100 is not rotated around the axis of the main journal, but it is rotated around the axis of the pin journal eccentric from the axis of the main journal, a large unbalance amount occurs not only to the crankshaft itself but also to the chuck 142 which supports the crankshaft. The unbalance amount changes at the time of half stroke conversion, but even if a cancel mechanism for the unbalance amount is provided, it is substantially impossible to eliminate the unbalance amount to an extent without a problem in practical use when the rotational frequency of the shaft is increased. From the above-described reason, an influence of rotation unbalance is exerted at the time of working, and harmful work displacement at the time of rotation and vibration and the like of the work or a mechanical system occur, thus making it impossible to obtain favorable working accuracy.
(2) When an extra-long shaft work such as a crankshaft is rotated and worked, it is absolutely necessary to increase flexural rigidity of the work against cutting load by performing auxiliary support (so-called rest) the shaft region at least at one spot having the same center of rotation as the region to be worked, in the vicinity of substantially the center in the axial direction, in addition to supporting of the work at its both ends to make heavy cutting possible or to obtain favorable finishing work accuracy. However, in the prior art, the crankshaft is rotated around the axis of the pin journal, and therefore there is no shaft portion which shares the center of rotation near the region to be worked, thus making it impossible to perform auxiliary support (rest) for it. Consequently, heavy cutting is difficult, and favorable finishing work accuracy cannot be expected.
(3) In the prior art, as described above, it is necessary to perform half stroke conversion, pin journal rotation angle phase indexing, cancel of the unbalance amount and the like inside the chuck, and therefore the structure of the chuck becomes very complicated, and expensive. Since the structure of the chuck is extremely complicated, there arises the problem that a trouble is easily caused, or the service life is short. Further, a number of hydraulic actuators are included in the chuck, and many hydraulic rotary joints are needed at a conduit line to supply oil to them, thus causing a fear of trouble of oil leakage occurring to seal regions of these hydraulic rotary joints.
Each of the prior arts in Japanese Patent Laid-open No. 8-25103, Japanese Patent Laid-open No. 11-90717 and the like adopts a method of milling the eccentric part (for example, the pin journal) of the crankshaft by moving the cutting edge of the rotated milling cutter in a predetermined locus from a diameter direction outside the shaft portion to be worked toward the shaft center, in a plane perpendicular to the axis of the eccentric part of the crankshaft that is the region to be worked. In this situation, a shoulder surface of the shaft portion to be worked is worked with the cutting edges of tips projected in a width direction of the cutter. However, since the cutter is rotating, in a corner portion at which the shoulder surface (journal side surface) and the shaft outer diameter surface intersect, it is impossible to work a so-called undercut groove, which is recessed in the axial direction, even if the cutter is moved in the axial direction. The crank shaft has many cases requiring i) grinding undercut at both ends in a width direction of the pin journal, or ii) a groove which is (undercut) recessed in the axial direction when R groove working is performed as pre-finishing of deep roll. However, for the above-described reason, the undercut groove working cannot be performed with working by means of the milling cutters, and it becomes necessary to add a lathe-cutting process step by a separate machine from the crankshaft miller.